Some multi-band or other tactical radios operate in the High Frequency (HF), Very High Frequency (VHF), and/or Ultra High Frequency (UHF) band for wireless communications. These transmit modes may be governed by the MIL-STD-188-141C standard, while data modulation/demodulation may be governed by the MIL-STD-188-110C standard, the disclosures which are incorporated by reference in their entirety. Alternatively, the transmit modes and data modulation/demodulation may be governed by other standards such as the NATO Standards STANAG 4538, STANAG 4539, or STANAG 4415, which are incorporated by reference in their entirety. An example multi-band tactical radio that may use HF, VHF, and UHF is the Joint Tactical Radio System (JTRS). The HF tactical radios as communications devices communicate long range at distances up to 3,000 Km or more as compared to VHF and UHF that commonly use short-range line-of-sight (LOS) communication links of typically 25 km or less. HF communications may require less infrastructure as compared to conventional, land mobile, cellular, and satellite voice and/or data communications systems that typically use a land-based infrastructure. HF radios are also rapidly deployable, and fixed base stations may provide command and control for mobile (vehicle mounted) and portable (manpack) users in the field. For that reason, many emergency and military communications devices have HF capability.
Systems providing wireless data communications services may employ link layer (Layer 2) protocols known as data link protocols, such as the NATO STANAG 5066 data link protocol or the data link protocols of NATO STANAG 4538, in order to deliver data with higher performance and reliability; these standards are incorporated by reference in their entirety.
In providing data communications services, these devices may use the Internet Protocol (IP) for packet construction and the Transmission Control Protocol (TCP) to enable devices to establish a connection, divide the payload data into data segments (also known as packets), deliver the data segments across the network, acknowledge data segments that have been successfully delivered, and ensure payload data segments are delivered in order. However, TCP may only work poorly, if at all, on some wireless links such as HF radio links because channel errors and congestion, as well as the waveform and protocol features intended to compensate for them, may cause long and variable data segment delivery latency, and result in spurious TCP retransmissions and, ultimately, transfer failures as TCP abandons the transfer due to excessive timeouts.
In TCP, the receiver device generates an acknowledgement back to the sender device when it has received a data segment (Hereafter, data segments may also be referred to as “packets,” “data packets,” “TCP segments,” or “TCP packets,” as will be understood by one skilled in the art). Upon receipt of the acknowledgement, the sender calculates corresponding the Round Trip Time (RTT, the time interval beginning when the segment is first sent and ending when the corresponding segment acknowledgement is received), and uses that RTT for determination of future timeouts. The TCP calculates the RTT to manage the data flow. In networks which include long-latency links (such as the wireless links discussed above), the TCP estimate of RTT may be sufficiently short (as compared to the actual RTT) so as to cause excessive retransmissions. The TCP RTT algorithm does not recalculate RTT based on acknowledgments that relate to retransmitted packets (since it is not possible to determine whether the acknowledgement is in response to the original transmission or one of the retransmissions), and thus retains an unrealistically short RTT estimate. Because the actual RTT is long, the TCP connections may not have time to adapt, and may thus fail, causing TCP connection timeouts, which ultimately result in communication failures.
TCP acceleration uses a TCP proxy as a performance enhancing proxy (PEP) and manages the interface in a more active manner. The TCP proxy intercepts IP packets containing TCP data segments as they are routed and masquerades as the TCP final destination (with respect to the sender) by sending simulated acknowledgements to the sender; the TCP proxy then transports the packets through the problematic link that is creating a delay, after which the packets are delivered to the ultimate destination. Even with a TCP proxy, however, wireless links such as HF links may still experience high error rates and high latency, in particular when operating under the STANAG 5066 standard that recommends transmitting for up to 120 seconds or more and then waiting for the acknowledgement before transmitting again. This creates an actual packet RTT of over two minutes, which may detrimentally impact the operation of the TCP protocol. There may also be high latency variability when a data packet is received in error and its latency may exceed four minutes or more as error rates rise. Adaptive data link protocols such as the STANAG 5066 data link protocol often respond by lowering the data rate for increased robustness, further increasing the RTT. In an HF data link, TCP timeouts may sometimes expire before a final acknowledgement is received. This may occur in the middle of a message if there were some packet errors on the communications link, while the backlog at the proxy is near a buffer maximum. Such timeouts may cause the TCP connection to fail, ultimately causing communications failures.